Separators of the kind under discussion have a helical trough or sluice which has an inner wall and an outer wall connected by a floor. In use, a pulp or slurry containing species to be separated is fed to the trough. The species in the slurry are sorted according to size and specific gravity with the largest and/or heaviest species moving to one side of the stream and the finer and/or lighter species being distributed in layers from the bed of the stream upwardly and from the inside of the curve outwardly and with water piling up on the outside of the bend. When the gravitational force is greater than the centrifugal force the largest and/or heaviest species are concentrated in a band near the inner wall ("concentrate band") and/or the finer and lighter species move towards the outer wall forming a band of depleted concentrate ("depleted band"). A splitter is arranged to remove the concentrate band via a take-off opening and the separation may be repeated on the depleted band.